1. Field of the Invention
The invention is related to the field of drill bits used to drill wellbores through earth formations. More specifically, the invention is related to types of nozzles, jets and other devices which affect the hydraulic properties of a drill bit.
2. Background Art
Drill bits used to drill wellbores through earth formations include, for example, fixed cutter bits, such as polycrystalline diamond compact (xe2x80x9cPDCxe2x80x9d) bits, and roller cone or xe2x80x9crockxe2x80x9d bits. Generally, these drill bits include a bit body which can be coupled to a drill string to rotate the bit, and various forms of cutting elements attached to the bit. PDC bits include PDC cutters affixed to the bit body, while roller cone bits include at least one roller cone rotatably mounted to the bit body. The roller cone includes cutting elements thereon, such as milled steel teeth or various forms of inserts.
Most of these drill bits include at least one, and typically a plurality of, xe2x80x9cnozzlesxe2x80x9d or xe2x80x9cjetsxe2x80x9d which are hydraulically coupled to the interior of the bit body. During drilling operations, as the drill bit is rotated, a drilling fluid (xe2x80x9cdrilling mudxe2x80x9d) is pumped through the interior of the drill string, where it is discharged through the jets. The drilling mud then travels upward through the annular space between the drill string and the wellbore. The drilling fluid cools and lubricates the cutting elements and the bit body, and cleans cuttings from the bottom of the wellbore as it is drilled. The drilling fluid also lifts the cuttings from the wellbore and transports them to the earth""s surface.
The number of, flow area (or orifice size) of, and placement of the jets on any particular drill bit depend on, among other factors, the hydraulic characteristics needed to drill a particular formation at a particular depth in a wellbore, and the type of bit being used. Typically, the wellbore operator desires to have a selected total flow area (xe2x80x9cTFAxe2x80x9d) of all the jets on the bit so that the drilling fluid circulation system will provide a selected pressure drop in the drilling fluid at a selected drilling fluid flow rate.
In certain circumstances, it is desirable to change the TFA of a bit during drilling of a particular wellbore. These circumstances may include, for example, that as the depth of the wellbore increases, the fluid pressure loss due to friction increases. Flow rates of the drilling fluid typically must be increased in order to maintain the necessary flow through the jets on the bit. More recently, specialized directional drilling tools, known as xe2x80x9crotary steerablexe2x80x9d systems have been developed for enabling wellbore operators to control the trajectory of the wellbore while rotating the drill string. When rotary steerable systems are used, an amount of pressure drop in the drill string may be limited by the pressure drop capacity of the rotary steerable system. In such cases, it is desirable to change the TFA of the drill bit to reduce fluid pressure drop along the entire drill string.
Changing TFA in a typical drill bit includes changing a flow area of one or more of the jets, or replacing a plug in a port therefore in the bit body with an orifice or jet. However, changing the TFA of the bit requires removing the entire drill string from the wellbore to make the jet, plug or orifice change. Removing the drill string can be expensive and time consuming. It is desirable to have a drill bit which can have the TFA changed during drilling without removing the bit from the wellbore.
One aspect of the invention is a drill bit which includes a bit body having a plurality of ports therein arranged to provide a flow path between an interior of a drill string and the exterior of the bit body. At least one flow relief is disposed in one of the ports. The at least one flow relief is adapted to provide an increase in total flow area of the bit upon application to the bit of a selected fluid flow condition.
Another aspect of the invention is a method for changing a total flow area of a drill bit, which includes pumping drilling fluid through the drill bit and operating a flow relief disposed in the bit to change the total flow area of the bit.